1. Field of the Invention
The invention relates generally to a device and method for steering a beam of light and, in particular, a wedge of electro-optic material having a variable refractive index causing beams of light passing through the wedge to be deflected by an angle dependent upon the refractive index.
2. Description of the Prior Art
A beam of light may be steered or deflected by diffracting the beam off a refractive index grating imposed upon a dielectric medium. The pitch and phase depth of the grating determines the diffraction angle, and the phase depth of the grating determines the diffraction efficiency. This technique is generally used in acousto-optic modulators and striped liquid crystal spatial light modulators. Although such diffraction is somewhat effective, it has the disadvantage of providing a low diffraction efficiency. In particular, the maximum efficiency is about 40% in the first order. In addition, diffraction into high orders must be blocked and such diffraction often causes high chromatic dispersion which restricts the use of such systems to narrow wavelength bands. In addition, such systems require highly complex drive electronics to vary the pitch for different steering angles.
One solution to the problem is the use of phase active liquid crystals trapped between optical flats to dynamically create optical wedges and lenses. Such devices are disclosed in co-pending, co-assigned U.S. patent application Ser. No. 08/192,589, filed Feb. 7, 1994, now abandoned titled Phase Active Diffractive Optics; Ser. No. 08/192,588, filed Feb. 7, 1994, now abandoned titled Phase Active Diffractive Optical Method; and Ser. No. 08/192,656, filed Feb. 7, 1994, now abandoned titled Phase Active Diffractive Device, the entire disclosures of which are incorporated herein by reference. Using principles of diffraction, a liquid crystal is spatially controlled to provide beam steering. These devices exploit the variable phase characteristics of the liquid crystal under the influence of different electrical fields. Various liquid crystals (e.g., nematic liquid crystals) exhibit birefringent properties. Therefore, these devices take advantage of the property of spatially controlling the liquid crystal to dynamically yield different optical devices, such as a lens or a wedge.
In addition, these devices may spatially address individual portions of the liquid crystal at high resolution in order to effect a change in its refractive index. For example, in order to address the liquid crystal in one dimension only, it is necessary to address about 50 line pairs/mm or 100 addresses per mm. For a 25 mm aperture (which is a reasonably small aperture), there would be 2,500 addresses. If the wedge angle is changed at a frequency of 100 hz, this results in 2.5.times.10.sup.5 addresses per second. In order to address the liquid crystal in two dimensions, there would be 625.times.10.sup.6 addresses per second. As a result, the control or the addressing of the liquid crystal may create a bottleneck.